Studies on an electroluminescent (EL) element have led to blue electroluminescence using a single crystal of anthracene in 1965, and then, an organic electroluminescent element of a bilayer structure consisting of a hole layer (NPB) and a light-emitting layer (Alq3) was proposed by Tang in 1987. Since then, studies have been directed toward the implementation of high efficiency and long lifespan in electroluminescent elements, suggesting a multilayer structure comprising an organic layer responsible for hole injection or hole transport, an organic layer responsible for electron injection or electron transport, and an organic layer responsible for combining a hole and an electron to induce electroluminescence. The introduction of multilayer structures has improved performance of organic electroluminescent elements to a level of commercialization. As a result, starting from radio display products for automobiles in 1997, the application of organic electroluminescence elements has been expanded to mobile information display devices and TV displays.
Requirements of enlargement and high resolution for displays are accompanied by the problems of high efficiency and long lifespan in organic electroluminescence elements in high resolution displays. Particularly, the high resolution that is achieved by forming more pixels in the same area reduces a luminescent area of organic electroluminescent elements, incurring a decrease in lifespan. This is one of the most important technical problem to be solved for organic electroluminescent elements.
In an organic electroluminescent element, the application of a current or voltage across two opposite electrodes induces the injection of holes from the anode and electrons from the cathode into an organic layer. The injected holes and electrons recombine with each other to generate excitons which then return to the ground state, emitting light. According to kinds of electron spin of the excitons formed, the organic electroluminescent elements may be classified into fluorescent light-emitting elements in which decay of singlet excitons contributes to the production of light through spontaneous emission and phosphorescent light-emitting elements in which decay of triplet excitons contributes to the production of light through spontaneous emission.
Electron spin of excitons formed by the recombination of electrons and holes may either be in a singlet state or a triplet state at a ratio of 25% singlet state:75% triplet state. Fluorescent light-emitting elements in which light is emitted by singlet exciton, theoretically does not exceed 25% in internal quantum efficiency and 5% in external quantum efficiency, based on the formation rate of a singlet excitons. Phosphorescent light-emitting elements in which light is emitted by triplet exciton exhibits emission efficiency four times as high as that of fluorescent light-emitting elements.
Although phosphorescent light-emitting elements are higher in emission efficiency than fluorescent light-emitting elements, as described above, on a theoretical basis, a host that meets the color purity of deep blue and the high efficiency required in blue phosphorescent light-emitting elements is underdeveloped so that blue fluorescent light-emitting elements rather than blue phosphorescent light-emitting elements have predominantly been employed in products thus far.
Studies for improving properties of organic electroluminescent elements have reported that the prevention of holes from diffusing into an electron transport layer contributes to the stability of elements. Materials such as BCP or BPhen have been suggested as a blocking material introduced between a light emission layer and an electron transport layer to increase the recombination rate of holes and electrons by preventing the diffusion of holes into the electron transport layer and by limiting holes within the light emission layer. However, being poor in oxidation stability and thermal durability, the derivatives such as BCP or BPhen were observed to reduce the lifespan of organic electroluminescent elements, and thus finally failed in commercialization. Further, such materials inhibit the movement of electrons to increase the driving voltage of the organic electroluminescent element.